An ultrasonic atomizer typically comprises an elongated metallic body having interposed piezoelectric (PZT) elements therein and a liquid feed tube extending axially through the body from a rear liquid inlet to a front tip element. Electrical excitation of the PZT elements (i.e., the transducer) generates mechanical compression waves along the axis of the atomizer structure. When the PZT elements are electrically driven at the self-resonant frequency of the structure (point of maximum admittance and zero phase), a maximum motion at the tip element is produced. If a suitable fluid is introduced to the tip element, via the liquid feed tube, and an adequate electrical drive is present to produce a maximum tip motion, the fluid will atomize (i.e., break into small particles and dislodge from the tip element). This atomizing process depends upon (1) a controlled flow of liquid, (2) sufficient electrical drive power, and (3) proper drive frequency to the transducer.
However, the effect of introducing fluid to the tip element of the atomizer contributes a significant, dynamic load impedance to the voltage and current drive requirements. The load impedance changes the self-resonant frequency of the atomizer and shifts the frequency of the transducer to a new operating point. For maximum power transfer, it is essential that the drive power to the transducer has a frequency which always corresponds to that of the atomizer/transducer self-resonant frequency. In addition, the resistive component of the load impedance requires that additional drive power at the new frequency be provided to the transducer in order to maintain operation of the atomizer. Therefore, the transducer drive circuit must adapt to the changing conditions imposed by the atomizing process as follows: (1) adjust the drive frequency to compensate for load change due to the dynamics of the atomizing fluid, and (2) adjust the drive power to maintain fluid atomization with minimum applied power.
The major design problems of known drive systems are associated with the derivation of techniques for providing appropriate adaptive frequency and power control. A standard drive circuit for automatically controlling the drive frequency includes a phase comparator which senses the phase difference between the voltage and current of the drive signal. by insuring that the drive voltage and current are in phase, the circuit enables the excitation frequency to always follow the new self-resonant frequency of the atomizer due to the load impedance of the fluid. An example of this type of drive circuit can be found in U.S. Pat. No. 2,917,691. However, such circuits are often complex, expensive and inefficient.